Vapor lock resistant hydraulic fluids

ABSTRACT

A composition suitable for use as a hydraulic fluid comprises: 
     (a) a lubricating oil preferably in an amount of from 20 to 80% by weight; 
     (b) a minor amount of oil-soluble borate ester; 
     (c) an amine which prevents deposition of oil-insoluble hydrolysis products of the borate ester; and optionally 
     (d) an oil soluble orthoester of the formula: ##STR1##  wherein R 7 , R 8 , R 9  and R 10  are specified groups.

This application is a continuation of application Ser. No. 188,517, filed Sept. 18, 1980, which is a continuation of Ser. No. 095,350 filed Nov. 19, 1979, which is a continuation of Ser. No. 875,611 filed Feb. 6, 1978, which is a continuation of Ser. No. 799,096 filed May 20, 1977, which is a continuation of Ser. No. 717,207 filed Aug. 24, 1977 which is a continuation of Ser. No. 549,096, July 8, 1975, all of these applications being now abandoned.

This invention relates to hydraulic fluids and particularly to mineral oil hydraulic fluids.

In hydraulic systems for which mineral oil or certain other fluids are specified as the operative fluid problems arise due to the presence of water. This water may be present as the result of condensation or inadequate drying prior to the filling of the system or subsequently, by diffusing through worn seals or flexible hoses. In either case the vapour lock temperature of the fluid can be reduced to the boiling point of water even by the presence of very minor amounts of water, e.g. of the order of 0.5%. This constitutes a serious deficiency with particularly dangerous consequences in those systems which are brake systems or central hydraulic systems including brake systems.

According to the present invention there is provided a composition suitable for use as a hydraulic fluid which comprises:

(a) a lubricating oil, preferably in an amount of from 20 to 80% by weight based on the total weight of the composition;

(b) a minor amount of oil-soluble borate ester, preferably of the general formula: ##STR2## wherein: (i) R¹, R² and R³ are the same or different and each is an aryl group or a straight or branched chain alkyl group or alicyclic group, preferably containing from 4 to 20 carbon atoms, or is a group of the formula --(R⁴ O)_(n) R⁵ wherein each R⁴ is the same or different and each is an alkylene group, preferably ethylene, propylene or butylene, R⁵ is an alkyl group, preferably containing from 1 to 18, more preferably 1 to 4, carbon atoms, or an aryl group, and n is an integer, preferably of from 1 to 10, more preferably of from 2 to 4;

or

(ii) R¹ and R² are the same or different and as defined above and R³ is a group of the general formula: ##STR3## wherein R¹ and R² are as defined above and R⁶ is an alkylene group, preferably containing at least 4, more preferably from 4 to 20, carbon atoms, or is an oxyalkylene radical of the formula --R⁴ --OR⁴)_(n) wherein R⁴ and n are as defined above;

or

(iii) R¹ is as defined above and R² and R³ together form the group ##STR4## wherein R¹ is as defined above;

(c) an amine which prevents deposition of oil-insoluble hydrolysis products of the borate ester; and optionally

(d) an oil soluble orthoester of the formula: ##STR5## wherein R⁷ is hydrogen or an alkyl group, preferably methyl or an aryl, alkaryl, or aralkyl group; R⁸, R⁹ and R¹⁰ are the same or different, preferably the same, and each is a straight or branched chain alkyl group, preferably containing from 4 to 20 carbon atoms, an aryl, alkaryl, or aralkyl group, or the group --(R¹¹ --O)_(m) R¹² wherein each R¹¹ is the same or different and each is an alkylene group, preferably ethylene or propylene; R¹² is an alkyl group, preferably containing from 1 to 20 carbon atoms, or an aryl, alkaryl or aralkyl group; and m is an integer of from 2 to 10.

It is required that the borate ester used in the composition of the invention should be oil-soluble and, in the case of trialkyl borates, oil solubility may be provided by selecting an ester made from straight chain alcohols containing less than 12 carbon atoms or from branched chain alcohols containing up to 24 carbon atoms. In the case of borate esters of the type derived from di- and polyoxyalkylene glycol ethers, those derived from di- and polyoxyethylene glycol ethers are generally insoluble unless at least one of the terminal ether groups is sufficient to solubilise the ester. Alternatively oil-solubility for this latter type of ester may be achieved by incorporating polyoxypropylene or higher polyoxyalkylene radicals into the molecule.

Examples of particularly useful borate esters include

tris (dipropylene glycol monomethyl ether) borate

tris (ethylene glycol monobutyl ether) borate

tris (triethylene glycol monobutyl ether) borate

tris (tripropylene glycol monomethyl ether) borate

tri n-decyl borate

tri (isotridecyl) borate

tri (2-ethyl hexyl) borate

tri (3,3-dimethyl butyl) borate

The amine used in the present invention should have a reasonably low vapour pressure consistent with providing a vapour lock temperature in excess of 120° C. The amine used will also depend upon the borate ester used. A simple test for determining whether a particular amine is suitable for preventing deposition of oil-insoluble hydrolysis products of a particular borate ester consists of dissolving the amine and borate in the selected lubricating oil, (in the amounts intended in the final hydraulic fluid), sealing the resulting fluid together with 0.5% by weight of water in a clear glass ampoule and heating at 100° C. for 24 hours and cooling. In this so called "ampoule" test, if the resulting solution is clear and bright then the combination is satisfactory.

Many amines have been found to be suitable, including primary, secondary and tertiary amines, especially those containing a total of at least 5 carbon atoms. Amines which have been found to be particularly useful with a wide range of borates include Primene 81 R and Primene JMT which are commercially available primary amines with two methyl groups on the alpha carbon atom.

Other amines which may be useful are Mannich bases formed by condensation of an amine and formaldehyde with a phenol previously alkylated with di or polyisobutylene; polyisobutenyl succinimides derived from di or polyamines; or amides derived from di or polyalkyl polyamines and polyisbutenyl substituted monocarboxylic acids.

The amounts of components (b), (c) and (d) (when present) may each vary over a wide range. However, it is preferred to use from 1 to 50%, more preferably from 1 to 20%, particularly from 5 to 10% by weight of the borate ester; from 1 to 50%, more preferably from 5 to 30%, particularly from 10 to 20% by weight of the orthoester; and from 0.5 to 20%, more particularly from 1 to 10%, by weight of amine, the percentages being based on the total weight of the composition.

In compositions containing orthoester, the ratio of orthoester to borate may vary, for example, from 10:1 to 1:10 by weight but in general more orthoester than borate will be used, the preferred ratio varying from 5:1 to 2:1 by weight.

The ratio of borate to amine will depend upon the nature of the two particular compounds used but it is preferred to use from 5:1 to 1:1 by weight.

The lubricating oil used as base fluid in the compositions of the invention is preferably a mineral oil but may also be a synthetic hydrocarbon oil, a synthetic carboxylic acid ester or mixture thereof, a siloxane or phosphate ester or other well known synthetic lubricant.

The invention will now be illustrated by the following Examples:

Example 1

    ______________________________________                                         Mineral oil blend   85%                                                        Tris(tripropylene glycol                                                                           10%                                                        monomethyl ether)borate                                                        tridecyl borate      3%                                                        Primene JMT          2%                                                        ______________________________________                                    

This composition has the following physical characteristics: Boiling point 257° C., Viscosity at -40° C. 1328cS, Viscosity index 218. The composition was subjected to the Markey vapour-lock test in the dry state and after contamination with varying amounts of water, with the following results:

    ______________________________________                                         Water present (wt. %)                                                                          dry    0.2%     0.5% 0.75%                                     Vapour lock temperature                                                                        253    238      178  140                                       (°C.)                                                                   ______________________________________                                    

The mineral oil alone with 0.5% water had a vapour lock temperature of 101° C.

Examples 2 to 32

Further blends comprising various combinations of borate ester and amine inmineral oil were formulated and subjected to the Gilpin vapour-lock test. Details of these blends and of the results obtained are given in Table 1.

The base fluid used in each case was a naphthenic mineral oil having the following characteristics:

    ______________________________________                                         Viscosity      130cS at -40° C., 3.5cS at 100° F.                               and 1.31cS at 210° F.                                    Pour point     <-70° F.                                                 Boiling Point  248° C.                                                  Flash Point (closed)                                                                          208° C.                                                  Aniline Point   76° C.                                                  ______________________________________                                    

The Gilpin vapour-lock test was conducted in a Gilpin apparatus and by the Gilpin method as described in S.A.E. Paper 710 253 entitled "Operating performance of motor vehicle braking systems as affected by fluid water content." The Gilpin vapour-lock temperature (VLT) was taken to be the temperature which corresponded with the appearance of 3 ml of bubbles.

Examples 33 to 47

Gilpin vapour lock temperatures were evaluated (in the manner described above) for a range of compositions containing different combinations of borate ester and lubricating oil base fluid, Primene JMT being used as theamine component in each case. Details of these compositions and of the results obtained are given in Table 2.

Example 48

    ______________________________________                                         Tris - (tridecyl)orthoformate                                                                         10%                                                     Tris(dipropylene glycol monomethyl-                                                                   10%                                                     ether)borate                                                                   Primene JMT             5%                                                     Mineral Oil            75%                                                     ______________________________________                                    

This blend conformed with the base oil requirements of Specification DTD 585. The Gilpin vapour-lock temperature was 177° C. after heating at 100° C. for 24 hours with 0.5% water in a sealed glass ampoule.

Example 49

    ______________________________________                                         Tris(tridecyl)orthoformate                                                                            20%                                                     Tris(dipropylene glycol monomethyl                                                                     5%                                                     ether)borate                                                                   Primene JMT             5%                                                     Mineral Oil            70%                                                     ______________________________________                                    

This blend also conformed to the base oil requirements to DTD 585 Specification and the Gilpin (3 ml) wet vapour-lock temperature was 203° C.

Example 50

    ______________________________________                                         Tris(tridecyl)orthoformate                                                                            20%                                                     Tris(dipropylene glycol monomethyl)                                                                    5%                                                     borate                                                                         Primene JMT             3%                                                     Mineral Oil            72%                                                     ______________________________________                                    

This blend also conformed to the base oil requirements of the DTD 585 Specification and the Gilpin (3 ml) wet vapour-lock temperature was 206° C.

Example 51

    ______________________________________                                         Tris(tridecyl)orthoformate                                                                            20%                                                     Tris(dipropylene glycol monomethyl                                                                    5%                                                      ether)borate                                                                   Tris(tridecyl)borate   2%                                                      Primene JMT            3%                                                      Mineral Oil            70%                                                     ______________________________________                                    

This blend conformed to the base oil requirements of the DTD 858 Specification and has a Gilpin (3 ml) wet vapour-lock temperature of 205.5° C.

Examples 52 to 81

Further blends containing orthoester were formulated from a range of different orthoesters and borate esters. In each case Primene JMT was usedas the amine component and the base fluid was the naphthenic mineral oil used in Examples 2 to 32. Samples of these blends were subjected to the Gilpin (3 ml) vapour-lock test (i) after reaction with 0.5% water at 100° C. for 24 hours and (ii) after subjection to a humidity test at a Relative Humidity (RH) of 80% and temperature of 22° C. substantially as described in the FMVSS 113 Specification but extended to a 5 day period and without a reference fluid. Also, the Rubber Swell properties of the test fluids with respect to nitrile rubber were determined by measuring the increase in volume of a 2.54 cm square, 2 mm thick nitrile rubber specimen in 50 mls of fluid at 120° C. for 3 days.

Details of these blends and of the results obtained are given in Table 3.

The abbreviations and commercial products referred to in Tables 1 to 3 are as follows:

    ______________________________________                                         DPM            dipropylene glycol monomethyl                                                  ether                                                           TPM            tripropylene glycol monomethyl                                                 ether                                                           PPG            polypropylene glycol                                            Primene 81 R and                                                                              commercially available primary                                  Primene JMT    amines with two methyl groups                                                  on the alpha carbon atom                                        Lubrizol 894 and                                                                              commercially available poly-                                    Hitec E 638    isobutenyl succinimides of                                      (Edwin Cooper) polyalkylene polyamines.                                        Empilan KS 3   commercially available mixture                                                 of triethyleneglycol mono                                                      ethers of C.sub.9 to C.sub.11 alcohols.                         Empilan KB 2   commerciall available mixture                                                  of diethylene glycol mono                                                      ethers of C.sub.12 to C.sub.14 alcohols.                        Burning Oil    a paraffinic heavy kerosene                                                    having a flash point of 260° F.,                                        a specific gravity of 0.82 and                                                 viscosities at 100° F. and 210° F.                               of 4.5cS and 1.6cS respectively.                                Refrigerant Oil A                                                                             a blend of naphthenic mineral                                                  oils having a specific gravity                                                 of 0.892, viscosity at 100° F.                                          of 48cS, flash point of 360° F.                                         and a pour point of -30° F.                              Refrigerant Oil B                                                                             a blend of naphthenic mineral                                                  oils having a specific gravity                                                 of 0.983, flash point of 330° F.                                        pour point of -30° F. and                                               viscosities at 100° F. and 210° F.                               of 53.4cS and 5.36cS respect-                                                  ively.                                                          Refrigerant Oil C                                                                             a commercially available                                                       refrigerant oil manufactured                                                   by British Petroleum under the                                                 trademark ZERICE 353 and                                                       believed to be a mixture of                                                    alkylated benzenes.                                             Silicone Fluid an experimental silicone brake                                                 fluid supplied by Union Carbide                                                Corporation.                                                    ______________________________________                                    

The vapour-lock test results set out in the foregoing Examples and in Tables 1 to 3 show that fluids in accordance with the invention retain unexpectedly high vapour-lock temperatures even in the presence of water. Furthermore, the rubber swell test results set out in Table 3 show that fluids in accordance with the invention may be blended so as to provide fluids having rubber swell properties acceptable in commercial hydraulic systems.

                                      TABLE I                                      __________________________________________________________________________     BORATE ESTER            AMINE                                                  Example               wt.             wt.                                      No                    %               %  GILPIN VLT (°C.)               __________________________________________________________________________     2    TETRA(DPM)PYROBORATE                                                                            10                                                                               PRIMENE 81 R  2  181                                   3    TETRA(TPM)PYROBORATE                                                                            10                                                                               LUBRIZOL 894  20 178                                   4    TRIS n-BUTYL BORATE                                                                             15                                                                               2-ETHYL HEXYLAMINE                                                                           2  133                                   5    TETRA(PENT 2YL)  10                                                                               PRIMENE JMT   5  170                                        PYROBORATE                                                                6    TRIS(DPM)METABORATE                                                                             10                                                                               PRIMENE 81R   10 186                                   7    TRIS(n-OCTYL)    10                                                                               PRIMENE JMT   10 155                                        METABORATE                                                                8    TRIS(BUTYL MONOGLYCOL)                                                                          10                                                                               PRIMENE JMT   5  167                                        METABORATE                                                                9    TRIHEXYLENE GLYCOL                                                                              10                                                                               PRIMENE JMT   2  143                                        BISBORATE                                                                 10   TRIS(OLEYL)BORATE                                                                               15                                                                               PRIMENE JMT   10 120                                   11   TRIS(m-TOLYL)BORATE                                                                             10                                                                               PRIMENE JMT   2  222                                   12   TRIS(p-TOLYL)BORATE                                                                             10                                                                               PRIMENE JMT   2  205                                   13   TRIS(o-TOLYL)BORATE                                                                             10                                                                               PRIMENE JMT   2  206                                   14   TRIS(p-tert.BUTYL                                                                               10                                                                               PRIMENE JMT   10 177                                        PHENYL)BORATE                                                             15   TRIS(BENZYL)BORATE                                                                              10                                                                               PRIMENE JMT   5  151                                   16   TRIS(n-OCTYL)BORATE                                                                             15                                                                               DIHEXYLAMINE  2  139                                   17   TRIS(PENT 2YL)   15                                                                               PRIMENE 81R   5  144                                        BORATE                                                                    18   TRIS(2 NAPHTHYL) 10                                                                               PRIMENE JMT   10 218                                        BORATE                                                                    19   HEXYLENE GLYCOL BIS                                                                             10                                                                               PRIMENE JMT   2  167                                        DPM BIS BORATE                                                            20   PPG 1200 BIS ISO-                                                                               15                                                                               PRIMENE JMT   10 138                                        TRIDECYL BIS BORATE                                                       21   TRIS(OCT 2YL)BORATE                                                                             15                                                                               PRIMENE JMT   7  179                                   22   TRIS(ISODECYL)BORATE                                                                            15                                                                               PRIMENE JMT   10 135                                   23   TRIS(ISOTRIDECYL)                                                                               15                                                                               PRIMENE JMT   10 131                                        BORATE                                                                    24   TRIS(ISO-OCTADECYL)                                                                             20                                                                               PRIMENE JMT   20 126                                        BORATE                                                                    25   TRIS(3 METHYL    15                                                                               TRIOCTYLAMINE 20 154                                        1 BUTYL)BORATE                                                            26   TRIS(3 METHYL PENT                                                                              15                                                                               PRIMENE JMT   5  171                                        3YL)BORATE                                                                27   TRIS(2 METHYL    15                                                                               PRIMENE JMT   5  171                                        CYCLOHEXYL)BORATE                                                         28   TRIS(BUTYL MONO- 15                                                                               DIALLYLAMINE  5  169                                        GLYCOL)BORATE                                                             29   TRIS(BUTYL TRIGLYCOL)                                                                           15                                                                               n-HEXYLAMINE  2  184                                        BORATE                                                                    30   TRIS(HEXYL-DIGLYCOL)                                                                            15                                                                               PRIMENE 81R   2  145                                   31   TRIS(DPM)BORATE  10                                                                               HITEC E 638   20 186                                   32   TRIS(TPM)BORATE  10                                                                               PRIMENE JMT   10 181                                   __________________________________________________________________________

                                      TABLE 2                                      __________________________________________________________________________     BORATE ESTER                                                                   Example                wt                                                                               PRIMENE               GILPIN                          No                     % JMT (wt %)                                                                            BASE FLUID     VLT (°C.)                __________________________________________________________________________     33   TRIS(n-BUTYL)BORATE                                                                              15                                                                               5      BURNING OIL    142                             34   TRIS HEXYLENE GLYCOL                                                                             10                                                                               5      BURNING OIL    157                                  BIS BORATE                                                                35   TRIS (BUTOXYETHOXY                                                                               15                                                                               5      BURNING OIL    166                                  PROPYL)BORATE                                                             36   TRIS(2 METHYL CYCLO-                                                                             15                                                                               5      DI(ISO-OCTYL)  220                                  HEXYL)BORATE               ADIPATE                                        37   TRIS(BUTYL MONO   15                                                                               2      REFRIGERANT OIL A                                                                             185                                  GLYCOL)BORATE                                                             38   TRIS(3 METHYL 1   15                                                                               5      REFRIGERANT OIL B                                                                             153                                  BUTYL)BORATE                                                              39   TRIS(o-BUTYL)     15                                                                               5      REFRIGERANT OIL C                                                                             236                                  BORATE/TRIS(O-TOLYL)                                                           BORATE                                                                    40   TRIS(n-BUTYL)BORATE                                                                              15                                                                               5      SILICONE FLUID 133                             41   TRIS(BUTYL MONO-  15                                                                               5      SILICONE FLUID 188                                  GLYCOL)BORATE                                                             42   TRIS(3 METHYL 1 BUTYL)                                                                           15                                                                               5      PENTAERYTHRITOL-                                                                              180                                  BORATE                     HEPTANOATE                                     43   TRIS(o-TOLYL)BORATE                                                                              10                                                                               5      TRIMETHYLOL PROPANE                                                                           256                             44   TRIS(BUTYL TRIGLYCOL)                                                                            15                                                                               5      DI (TRIDECYL)  223                                  BORATE                     DODECANE DIOATE                                45   TRIS(DPM)BORATE   10                                                                               5      DI(2-ETHYLHEXYL)                                                                              218                                                             DODECANE DIOATE                                46   TRIS(PHENYL GLYCOL ETHER)                                                                        10                                                                               2      TRIBUTYL PHOSPHATE                                                                            205                                  BORATE                                                                    47   TRIS(EMPILAN KS 3)                                                                               10                                                                               2      TRI BUTYL PHOSPHATE                                                                           195                                  BORATE                                                                    __________________________________________________________________________

                                      TABLE 3                                      __________________________________________________________________________     ORTHOESTER                  BORATE ESTER                                       Example                 wt                     wt                              No   Name               %   NAME               %                               __________________________________________________________________________     52   TRIS(BUTYL)ORTHOFORMATE                                                                           25  TRIS(n-DECYL)ORTHOBORATE                                                                          12                              53   TRIS(BUTYL)ORTHOFORMATE                                                                           15  TRIS(n-OCTYL)ORTHOBORATE                                                                          5                               54   TRIS(2 ETHYL HEXYL)                                                                               28  TRIS(n-BUTYL)ORTHOBORATE                                                                          7                                    ORTHOFORMATE                                                              55   TRIS(2 ETHYL HEXYL)                                                                               25  TRIS(TRIDECYL)ORTHOBORATE                                                                         20                                   ORTHOFORMATE                                                              56   TRIS(BUTYL TRIGLYCOL)                                                                             30  TRIS(ISO DECYL)ORTHOBORATE                                                                        11                                   ORTHOFORMATE                                                              57   TRIS(DOWANOL DPM)  15  TRIS(METHYL BUTYL) 13                                   ORTHOFORMATE           ORTHOBORATE                                        58   TRIS(TRIDECYL)     30  TRIS(PENT-2-YL)    15                                   ORTHOFORMATE           ORTHOBORATE                                        59   TRIS(TRIDECYL)     15  TRIS(2-OCTYL)ORTHOBORATE                                                                          13                                   ORTHOACETATE                                                              60   TRIS(TRIDECYL)     25  TRIS(OLEYL)ORTHOBORATE                                                                            15                                   ORTHOFORMATE                                                              61   TRIS(OLEYL) ORTHOFORMATE                                                                          15  TRIS(p-TOLYL)ORTHOBORATE                                                                          3                               62   TRIS(DPM)ORTHOACETATE                                                                             10  TRIS(BUTYL MONOGLYCOL)                                                                            6                               63   TRIS(BUTYL TRIGLYCOL)                                                                             30  TRIS(BUTYL TRIGLYCOL)                                                                             17                                   ORTHOFORMATE           ORTHOBORATE                                        64   TRIS(DPM)ORTHOFORMATE                                                                             15  TRIS(HEXYL DIGLYCOL)                                                                              4                                                           ORTHOBORATE                                        65   TRIS(OLEYL)ORTHOFORMATE                                                                           20  TRIS(DPM)ORTHOBORATE                                                                              5                               66   TRIS(BENZYL)ORTHOFORMATE                                                                          20  TRIS(BUTYL MONOGLYCOL)                                                                            5                                                           ORTHOBORATE                                        67   TRIS(ISO OCTADECYL)                                                                               25  TETRA(TPM)PYBORATE 4                                    ORTHOFORMATE                                                              68   TRIS(DPM)ORTHOACETATE                                                                             17  TRIS(DPM)METABORATE                                                                               4                               69   TRIS(ALLYL)ORTHOFORMATE                                                                           19  TRIS(BENZYL)ORTHOBORATE                                                                           7                               70   TRIS(BUTYL MONOGLYCOL)                                                                            25  TRIS(METHYL CYCLOHEXYL)                                                                           10                                   ORTHOACETATE           ORTHOBORATE                                        71   TRIS(TRIDECYL)ORTHO-                                                                              15  TRIS(TPM)ORTHOBORATE                                                                              2                                    FORMATE                                                                   72   TRIS(BUTYL MONOGLYCOL)                                                                            30  TETRA(DPM)PYROBORATE                                                                              3                                    ORTHOACETATE                                                              73   TRIS(ISO OCTADECYL)                                                                               27  HEXAMETHYLENE GLYCOL BIS                                                                          9                                    ORTHOFORMATE           DPM. BIS BORATE                                    74   TRIS(2 ETHYL HEXYL)                                                                               22  TETRA(BUTYL MONOGLYCOL)                                                                           5                                    ORTHOFORMATE           PYROBORATE                                         75   TRIS(EMPILAN KB2)  14  TRIS(EMPILAN KB2)  5                                    ORTHOFORMATE           ORTHOBORATE                                        76   TRIS(DPM)ORTHOACETATE                                                                             23  TRIS(ISOOCTADECYL) 6                                                           ORTHOBORATE                                        77   TRIS(BUTYL MONOGLYCOL)                                                                            21  TRIS(p-tert.BUTYL  8                                    ORTHOACETATE           PHENYL)ORTHOBORATE                                 78   TRIS(BUTYL TRIGLYCOL)                                                                             24  TRIS(2 NAPHTHYL)   9                                    ORTHOFORMATE           ORTHOBORATE                                        79   TRIS(DPM)ORTHOFORMATE                                                                             14  PPG 1200 BIS TRIDECYL                                                                             3                                                           BIS BORATE                                         80   TRIS(3 METHYL PENTYL)                                                                             15  TRIS(TPM)ORTHOBORATE                                                                              4                                    ORTHOFORMATE                                                              81   TRIS(TRIDECYL)ORTHO-                                                                              15  TRIS(DPM)ORTHOBORATE                                                                              2                                    VALERATE                                                                  __________________________________________________________________________                                              GILPIN VLT                                                          GILPIN VLT AFTER 5 DAY                                             NITRILE RUBBER                                                                             AFTER REACTION                                                                            HUMIDITY TEST                         Example                                                                             AMINE        VOLUME SWELL (%)                                                                           WITH 0.5% WATER                                                                           80% RH 22° C.                  No   NAME     wt %                                                                               (3 days at 120° C.)                                                                 (°C.)                                                                              (°C.)                          __________________________________________________________________________     52   PRIMENE JMT                                                                             5   1.55        144        128                                   53   PRIMENE JMT                                                                             6   3.5         134        148                                   54   PRIMENE 81R                                                                             2   0.7         161        154                                   55   PRIMENE JMT                                                                             10  0.9         198        203                                   56   PRIMENE JMT                                                                             10  12.4        195        138                                   57   PRIMENE JMT                                                                             10  5.1         172        130                                   58   PRIMENE JMT                                                                             12  -1.6        164        120                                   59   PRIMENE JMT                                                                             8   3.4         184        115                                   60   PRIMENE JMT                                                                             10  3.5         185        133                                   61   PRIMENE JMT                                                                             3   11.3        --         221                                   62   PRIMENE JMT                                                                             2   6.1         152        120                                   63   PRIMENE 81R                                                                             3   19.8        168        137                                   64   PRIMENE UMT                                                                             1   8.8         174        211                                   65   PRIMENE JMT                                                                             3   2.5         129        156                                   66   PRIMENE JMT                                                                             5   32.9        199        196                                   67   PRIMENE JMT                                                                             2   -0.2        154        249                                   68   PRIMENE JMT                                                                             10  13.3        171        175                                   69   PRIMENE JMT                                                                             4   17.1        120        110                                   70   PRIMENE JMT                                                                             5   7.8         193        155                                   71   PRIMENE JMT                                                                             2   0.3         147        242                                   72   PRIMENE JMT                                                                             6   9.7         197        202                                   73   PRIMENE JMT                                                                             2   0.2         214        218                                   74   PRIMENE JMT                                                                             5   4.5         178        144                                   75   PRIMENE JMT                                                                             3   2.9         126        133                                   76   PRIMENE JMT                                                                             3   8.0         179        207                                   77   PRIMENE JMT                                                                             8   23.1        205        149                                   78   PRIMENE JMT                                                                             9   35.6        204        172                                   79   PRIMENE JMT                                                                             5   6.0         166        212                                   80   PRIMENE JMT                                                                             3   11.3          112.5    123                                   81   PRIMENE JMT                                                                             2   1.9          127*      149                                   __________________________________________________________________________     *With 0.25% Water                                                          

We claim:
 1. A composition of matter suitable for use as a hydraulic fluid which comprises(a) at least 20%, by weight, of a lubricating oil which is a member selected from the group consisting of a mineral oil, a synthetic hydrocarbon oil, a synthetic carboxylic acid ester, a siloxane or a phosphate ester; (b) 1 to 50%, by weight, of oil-soluble borate ester which is a compound, or mixture of compounds, having the formula: ##STR6## wherein: (i) R¹, R² and R³ are the same or different and each is an aryl group or a straight or branched chain alkyl group or alicyclic group containing from 4 to 20 carbon atoms, or is a group of the formula --R⁴ O)_(n) R⁵ wherein each R⁴ is the same or different and each is an alkylene group selected from ethylene, propylene or butylene, R⁵ is an alkyl group containing from 1 to 18 carbon atoms, or an aryl group, and n is an integer, from 1 to 10; or(ii) R¹ and R² are the same or different and as defined above and R³ is a group of the general formula: ##STR7## wherein R¹ and R² are as defined above and R⁶ is an alkylene group containing from 4 to 20 carbon atoms, or is an oxyalkylene radical of the formula --R⁴ --OR⁴)_(n) wherein R⁴ and n are as defined above; or (iii) R¹ is as defined above and R² and R³ together form the group ##STR8## wherein R₁ is as defined above; (c) 0.5 to 20%, by weight, of an amine which prevents deposition of oil insoluble hydrolysis products of the borate ester; and (d) 1 to 50%, by weight, of an oil soluble orthoester of the formula: ##STR9## wherein R⁷ is hydrogen or lower alkyl; R⁸, R⁹ and R¹⁰ are the same or different and each is a straight or branched chain alkyl containing from 4 to 20 carbon atoms, benzyl or the group --(R¹¹ --O)_(m) --R¹² where R¹¹ is a lower alkylene group and R¹² is an alkyl group containing 1 to 20 carbon atoms; and m is an integer from 2 to
 10. 2. A composition as claimed in claim 1 wherein R⁷ is a hydrogen atom or a methyl group.
 3. A composition as claimed in claim 1 which contains from 5 to 30% by weight of orthoester based on the total weight of the composition.
 4. A composition as claimed in claim 1 wherein the ratio of orthoester to borate ester is in the range from 5:1 to 2:1 by weight.
 5. A composition as claimed in claim 1 containing from 1 to 20% by weight of borate ester based on the total weight of the composition.
 6. A composition as claimed in claim 1 containing from 1 to 10% by weight of amine based on the total weight of the composition.
 7. A composition as claimed in claim 1 wherein the ratio of borate ester to amine is from 5:1 to 1:1 by weight. 